A flashing SES indicator means imminent catalytic converter damage if allowed to run continuously. Permanent damge can occur in less than 50 miles of driving with a flashing SES light. P0300 means a general misfire condition while P0304 means its cylinder #4 misfiring. Replacing plugs and ignition control module eliminated two parts leaving the cylinder head and piston/rings to check. A compression test would help to determine mechanical integrity while a clogged cat may be another possibility. The test for a clogged cat would be to remove the front O2 sensor and use a pressure gauge to measure exhaust back pressure; less than 1 psi is considered normal while more means the converter is clogged and blocking exhaust flow. Some members in the S-series forums check for clogged catcons by leaving the O2 sensor out and driving briefly; this alternate LOUD exhaust path sometimes improves engine response to verify a blocked catcon. Another method would be to bang the catcon with your hand and listen for loose cat guts; catcons aren't supposed to have any loose cat guts as the substrate is a molded high temperature honey comb passage way to allow free exhaust flow while converting pollutants to harmless byproducts.

A clogged fuel filter may strangle fuel flow while lowering pressure and leaning out fuel mixtures to cause high temperatures that can result in misfires, melted parts, excessive high cat temps. A fuel pressure check can eliminate a fuel issue. Normal pressure, 50-60 psi.

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A flashing SES indicator means imminent catalytic converter damage if allowed to run continuously. Permanent damge can occur in less than 50 miles of driving with a flashing SES light. P0300 means a general misfire condition while P0304 means its cylinder #4 misfiring. Replacing plugs and ignition control module eliminated two parts leaving the cylinder head and piston/rings to check. A compression test would help to determine mechanical integrity while a clogged cat may be another possibility. The test for a clogged cat would be to remove the front O2 sensor and use a pressure gauge to measure exhaust back pressure; less than 1 psi is considered normal while more means the converter is clogged and blocking exhaust flow. Some members in the S-series forums check for clogged catcons by leaving the O2 sensor out and driving briefly; this alternate LOUD exhaust path sometimes improves engine response to verify a blocked catcon. Another method would be to bang the catcon with your hand and listen for loose cat guts; catcons aren't supposed to have any loose cat guts as the substrate is a molded high temperature honey comb passage way to allow free exhaust flow while converting pollutants to harmless byproducts.

A clogged fuel filter may strangle fuel flow while lowering pressure and leaning out fuel mixtures to cause high temperatures that can result in misfires, melted parts, excessive high cat temps. A fuel pressure check can eliminate a fuel issue. Normal pressure, 50-60 psi.

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Yes, it can be either boots or ignition coil. Substituting can work and be expensive to find out so why not test them out in the open by laying them out on the engine for a spark test? All this requires is removing the icm, ign coil pack, plug boots and plugs for reassembly together on top of the engine and connected to the wiring harness with the plug bases grounded to the engine block. Having someone turn the ignition key to START will have the starter spinning while you observe for spark across all four plugs. Done correctly, all plugs should display spark jumping the plug gaps. The engine will spin faster since no compression is developed to allow faster spark indication. Any missing spark from any plug would suggest either a missed connection somewhere or a faulty coil pack.

Remove the fuel pump fuse to disable fuel injection.

A faulty ignition coil pack may reveal a small hair line crack in the housing where carbon tracking indicates the least path of resistance for high voltage to travel on its way to the nearest ground - through the epoxy encapsulation, plastic, plug boots, etc., instead of traveling through the plugs center electrode and jump the plug gap to the side electrode that's grounded to the engine. A faulty main connection may have a broken terminal, internally or externally resulting in missing spark to one or several coils.

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Almost 112k miles. I've replaced spark plugs and ignition control module. Still stumbling and lack of power.

I searched thru the forums and seems like there might be a couple of common issues: clogged pre-cat and grounding problem. Where do I check the grounds?

What should I replace next?? The ignition coil and spark plug boots???

When mine ran but had no power, misfired and threw the P0300 code it turned out to be the coil pack. $93, simple replacement and fixed the issues. I cleared all the warning lights with my scanner but one would not reset so had to disconnect the battery cable for a few minutes, then connected it back up and all has been fine for the last 2500 miles since the repair.

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